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llama: re-create the KV cache when flash attention resolves to disabled (performance and layout bug) #26460

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llama: re-create the KV cache when flash attention resolves to disabled (performance and layout bug) #26460

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38 changes: 30 additions & 8 deletions src/llama-context.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -378,15 +378,15 @@ llama_context::llama_context(
}

// init the memory module
if (!hparams.vocab_only) {
llama_memory_params params_mem = {
/*.type_k =*/ params.type_k,
/*.type_v =*/ params.type_v,
/*.swa_full =*/ params.swa_full,
/*.ctx_type =*/ cparams.ctx_type,
/*.mem_other =*/ llama_get_memory(cparams.ctx_other),
};
const llama_memory_params params_mem = {
/*.type_k =*/ params.type_k,
/*.type_v =*/ params.type_v,
/*.swa_full =*/ params.swa_full,
/*.ctx_type =*/ cparams.ctx_type,
/*.mem_other =*/ llama_get_memory(cparams.ctx_other),
};

if (!hparams.vocab_only) {
memory.reset(model.create_memory(params_mem, cparams));
}

Expand Down Expand Up @@ -453,8 +453,30 @@ llama_context::llama_context(
LLAMA_LOG_INFO("%s: pipeline parallelism enabled\n", __func__);
}

const bool flash_attn_requested = cparams.flash_attn;

sched_reserve();

// The memory module above was created with attn_v_trans = !flash_attn, but under
// LLAMA_FLASH_ATTN_TYPE_AUTO that read the *requested* value: the probe inside
// resolve_fused_ops (reached via sched_reserve) may only have turned flash attention
// off afterwards. When it does, V was laid out for flash attention while attention
// will actually run unfused, and the unfused path then needs a transposed V - so ggml
// inserts a cont(transpose(v)) per layer per token, which is expensive at depth.
//
// Re-create the memory module so the layout matches the resolved value. This can only
// trigger on the first reserve, since resolve_fused_ops clears cparams.auto_fa, and at
// this point the cache is still empty, so no cached data is moved or lost.
if (memory && flash_attn_requested && !cparams.flash_attn) {
LLAMA_LOG_INFO("%s: flash attention resolved to disabled - re-creating the memory "
"module so the V cache is transposed for the unfused path\n", __func__);

memory.reset(model.create_memory(params_mem, cparams));

sched_need_reserve = true;
sched_reserve();
}

if (!cparams.flash_attn) {
if (ggml_is_quantized(params.type_v)) {
throw std::runtime_error("quantized V cache was requested, but this requires Flash Attention");
Expand Down